MOVEMENTS DUE TO SWELLING, ETC. 409 



1900) to be of general occurrence in all bodies capable of swelling both in the 

 cell-membrane and in the protoplasm. He lays stress on the fact that the 

 diameter of the spaces is uniform, amounting to about one fx. Certainly, in 

 protoplasm these very minute but visible cavities show all transitions to the 

 large vacuoles. Butschli holds that the cavities contain a dilute solution of the 

 swellable body which is concentrated by loss of water and thus acts osmotically. 

 The expansion of the walls of the alveoli would thus be due to osmotic pressure. 

 In addition to other difficulties this conception is open to the criticism that cel- 

 lulose is quite insoluble in water, Butschli does not, however, hold that during 

 the process of swelling the stretching of the walls of the cavities is due only to 

 the pressure of their contents ; he expressly shows that water is also absorbed 

 by the walls of the cavities themselves, a point in which substances capable of 

 swelling differ from those incapable of doing so ; at the same time it must be 

 remembered that the latter also may possess a honeycomb structure. The 

 absorption of water by the walls of the cavities is considered by Butschli to 

 be a chemical phenomenon, a case of hydration in fact, and he thinks that this 

 water cannot be got rid of merely by pressure, and, further, that the water 

 expressed in Reinke's experiments above described was only water from the 

 cavities of the honeycomb and from the larger spaces in the substance. We 

 may, however, quite well make use of Nageli's physical hypothesis for the 

 imbibition of water by the walls of these cavities and so combine his theory 

 with that of Butschli. Hence it is worth noting that (see p. 407) the existence 

 of intermicellar spaces, corresponding to Butschli's alveoh, had already been 

 considered by Nageli (1858, 342). 



No matter which theory be the correct one, the walls of the cavities must 

 be increased by the imbibition, and the cavities must thus be able to hold more 

 water, itself out of reach of the attractive force of the micellae. Butschli's 

 observation that in the process of drying the walls of the cavities collapse and 

 approach each other until the lumina entirely disappear is of the utmost im- 

 portance. The full significance of the disappearance of the alveolar structure in 

 drying and its reappearance on water being once more absorbed will become 

 evident later on when we have studied the phenomena of cohesion (p. 417). 



The alterations in volume associated with swelling and shrivelling permits 

 of the execution of movements on the part of such bodies, and this leads us 

 back to the hygroscopic movements we started with. If the object under 

 consideration is capable of swelling equally in all directions, then it or its 

 parts will be able to exhibit movements only in straight lines, but such 

 movements are of no further interest. The bending, twining, and twisting 

 of hygroscopic organs can obviously be produced only if the capacity for 

 swelling varies in different directions, when layers with greater powers of imbibi- 

 tion stand in antagonism to those with less capacity for swelling. We distinguish 

 the layer which contracts most as the ' contractile ' or ' dynamical ' layer, and 

 that which does so least as the 'resistant' layer. Variations in the capacity for 

 swelling are due, in the first instance, to the varied nature of the material, in 

 the present instance the cell-wall, generally put down to chemical differences 

 but assumed by Nageli to be physical, and especially dependent on the vary- 

 ing size of the micellae. On the other hand, the structure of the membrane 

 may render possible differences in capacity for swelling in different directions. 

 Nageli's micellar theory, as also Butschli's alveolar theory, equally well explain 

 such unequal swelling. We will consider the observations themselves without 

 going into theories with regard to them, and we find, speaking quite generally, 

 that a cell which is not isodiametric is unequally extensible in its three chief 

 space dimensions. The greatest capacity for swelling in an elongated cell is in 

 a radial direction, i.e. at right angles to the concentric layers of which its wall is 

 composed ; it has less capacity for swelling tangentially, and least of all longi- 



